Field of the Invention
The present invention relates to a memory control apparatus for use in a CRT controller which is used as a receiving terminal for TELETEXT, VIDEOTEX, or the like.
Recently, TELETEXT and VIDEOTEX, in which characters and graphics representing a variety of useful information are transmitted to users through a transmission medium, have been developed in many countries. Particularly, in Canada and the USA, NAPLPS (North American Presentation Level Protocol Syntax) has been standardized as a presentation level protocol. An image is resolved into basic graphical elements such as a point, line, and arc which are coded and sent together with the coordinate data. This system is generally called an alpha geometric system.
In this field, a new system has been developed in which a picture description instruction (PDI) is used for the picture data transmission. PDI contains instructions for drawing the basic graphical elements on a CRT screen and instructions for designating the color of a picture. The picture information is expressed using combinations of various PDIs. To process the PDI signal, a microprocessor (MPU) is used. The MPU, upon receipt of the PDI signal, distingushes the type of a picture to be drawn and reads out a processing routine necessary for drawing the picture from memory. In the picture processing routine, the data which is included in the PDI is used to specify a coordinate position on the screen. The coordinate position of a picture element (abbreviated to "pel") corresponding to a specific address in a buffer memory on a locus of the picture is calculated by a programmed algorithm. The data is written into the buffer memory. This process is repeated, and the picture is drawn. When the drawing positions are the same as positions which were previously determined, the previous image data is replaced by the present image data in the buffer memory. After the data processing of various PDIs, the pictorial information is generated. The PDI receiving terminal for drawing a picture by this processing thus needs a picture memory control circuit which can write the data in units of a display element to the buffer memory.
In the PDI receiving terminal described above, the thickness of a line in a picture is determined by the concrete thickness of one pel. Therefore, in order to draw a picture with a thick line, it is necessary to transmit a PDI many times to displace drawing points on the CRT screen. However, this repetitive transmission of instructions deteriorates the data transmission efficiency.
To solve the above problem, there is a proposal in which a command to specify a logical pel is additionally contained in the transmitted PDI. The logical pel defines the thickness of a line in drawing line figures.
A command signal for specifying a logical gel contains signals representing a horizontal size (dX) and a vertical size (dY) of the logical pel. dX and dY are integer multiples of one pel. When the command signal specifying the logical pel is received, the MPU sets a write address designating a start point in the buffer memory. The data to be written in the address designating the start point is contained in the PDI. Then, the MPU writes the data of one pel into the write address and updates the write address to fill the logical pel (thickness) with the horizontal size (dX) and the vertical size (dY) as specified. Every time the write address is updated, the MPU writes the one pel of data into the updated write address. The updated processing of the write address and the writing of one pel of data are alternately performed as designated by software in the MPU. Therefore, as the horizontal size (dX) and the vertical size (dY) of the pel transmitted becomes larger, the data writing time to fill the size of the pel is longer.
The logical pel processing function provides a substantially constant transmitting time for pictorial data regardless of the sizes of the logical pel lines. On the other hand, the time necessary for processing one PDI is increased as the thickness of the drawing line by the logical pel is thicker, for the data write processing to the buffer memory by the MPU takes longer. As a result, there often occurs a case that the PDI processing is not completed when the next PDI is received. The receiving terminal thus needs a great capacity of memory for storing the received data (PDI). Additional disadvantages are that the software of the MPU is complicated and that the data processing load of the MPU is large.